Agents that inhibit cell growth are potentially useful for treating cancer. One such agent is curacin A. Originally purified as a major lipid component of a strain of the cyanobacterium Lyngbya majuscula isolated in Curacao, curacin A is a potent inhibitor of cell growth and mitosis, binding rapidly and tightly at the colchicine site of tubulin. See, U.S. Pat. No. 5,324,739 to Gerwick, which is incorporated herein by reference.
Although antimitotic agents, both natural products and synthetic compounds, display a wide structural diversity, virtually all of them interact with the .alpha./.beta.-tubulin dimer, the major component of microtubules. Hamel's "Interactions of Tubulin with Small Ligands, " Microtubule Proteins, (CRC Press, 1990) and "Antimitotic Natural Products and Their Interactions with Tubulin," Med. Res. Rev., 16:207-231 (1996). Most of these compounds inhibit microtubule assembly in cells and in cell-free systems. A major mechanism involved in the cytotoxic action of these drugs appears to be altered microtubule dynamics, and most drugs studied thus far reduce tubulin turnover at microtubule ends.
Net inhibitors of microtubule assembly largely fall into two classes. The first group consists of a variety of complex natural products that inhibit the binding of vinca alkaloids to tubulin and inhibit formation of an intra-.beta.-tubulin cross link between cys12 and cys201/211. These natural products also interfere with GTP/GDP exchange on .beta.-tubulin (vinca domain agents).
The second group of net inhibitors of microtubule assembly consists of numerous synthetic compounds and structurally simpler natural products, such as the cis-stilbene combretastatin A-4 and the estrogen metabolite 2-methoxyestradiol. These compounds inhibit the binding of colchicine to tubulin, inhibit formation of an intra-.beta.-tubulin cross link between cys239 and cys354, have no effect on GTP/GDP exchange, and generally induce a GTPase reaction uncoupled from assembly (colchicine-site agents). A recurring structural theme in the colchicine-site agents has been at least one, and generally two aromatic, domains. Hamel, supra; and Luduena et al., "Tubulin Sulfhydryl Groups as Probes and Targets for Antimitotic and Antimicrotubule Agents," Pharmac. Ther., 49:133-152 (1991).
Curacin A is a potent colchicine-site antimitotic agent, and is a major exception to structural generalizations stated above in that it has no aromatic residue. The compound inhibits microtubule assembly and, despite its unique structure, is a potent competitive inhibitor of the binding of colchicine to tubulin. Blokhin et al., "Characterization of the Interaction of the Marine Cyanobacterial Natural Product Curacin A with the Colchicine Site of Tubulin and Initial Structure-Activity Studies with Analogs," Mol. Pharmacol., 48:523-531 (1995). Initial studies demonstrate that curacin A stimulated the uncoupled GTPase reaction typical of colchicine-site agents, and indirect observations were consistent with curacin A binding rapidly and dissociating slowly from tubulin. Id. Curacin A inhibits formation of the cys239-cys354 cross link in ,.beta.-tubulin, Luduena et al., "Interaction of Curacin A with Bovine Brain Tubulin," Mol. Biol. Cell, 5:283a (1994). Moreover, curacin A also may have a relatively unusual effect on microtubule dynamics, in that low concentrations of the drug increase tubulin turnover at microtubule ends. Pack et al., "Curacin A, a New Potent Antimitotic Marine Natural Product, Increases Dynamic Instability of Microtubules, " Proc. Amer. Assoc. Cancer Res., 36:455 (1995).
Gerwick's U.S. Pat. No. 5,324,739 and the references cited above demonstrate that curacin A is an important, biologically active material. Curacin A has, however, exhibited limited chemical stability, and appears to undergo chemical modification to a less biologically active compound when stored neat. See, "Note Added in Proof" at page 1245 of Gerwick et al., "Structure of Curacin A, a Novel Antimitotic, Antiproliferative, and Brine Shrimp Toxic Natural Product from the Marine Cyanobacterium Lyngbya majuscula," J. Org. Chem., 1243-1245 (1994). Additional information, therefore, is needed about how to stabilize the biologically active curacin A structure, and likely the curacin B-curacin D structures as well. Additional information also is needed concerning the structure-activity characteristics of the naturally occurring curacins A-D, and analogs thereof.